High pressure fuel systems for diesel engines are known in the art. An example of such a fuel system is disclosed and claimed in U.S. Pat. No. 5,042,445 to Peters et al. and assigned to Cummins Engine Company. This patent discloses a cam driven unit injector having a pump that provides very high injection pressures (30,000 psi or higher) even at low engine speeds. Such high pressures promote better fuel vaporization during injection of the fuel in the cylinders thereby helping to complete combustion and thus reduce emissions in the engine exhaust. In view of the implementation of government regulations requiring reduced emissions in the engine exhaust, there is a considerable interest in the engine manufacturing industry to develop and refine such high pressure fuel systems.
While such systems have proven their practicality in the field, the applicants have noted an aspect of these systems which could bear improvement. Specifically, all high pressure fuel systems of which the applicants are aware of utilize a conventional fuel distribution valve for sequentially directing a pulse of pressurized diesel fuel to the particular fuel injector associated with the combustion cylinders of the engine. Such distribution valves include a fuel distribution shaft that is rotatably mounted in a bore in the valve body. The fuel distribution shaft includes axially spaced fuel inlet and outlet ports along one of its sides. These ports communicate with one another by means of an axial bore in the shaft. When the shaft is inserted into the valve body, the fuel inlet port communicates with an annulus in the valve body that in turn is connected to the output of a high pressure pump that generates the pressurized pulses of diesel fuel. The fuel outlet port of the shaft is sequentially registrable with a plurality of fuel distribution passages whose inlets are angularly spaced around the inner diameter of the shaft receiving bore in the valve body. These passages diverge from the bore in the valve body like spokes and ultimately communicate with the fuel injectors that feed vaporized fuel into the combustion cylinders. In operation, the fuel distribution shaft is linked to the crankshaft of the diesel engine so as to continuously rotate along with the crankshaft. The pressurized pulses of fuel are generated as the fuel distribution port of the rotating shaft comes into registration with one of the fuel receiving passages in the valve body in order to sequentially transfer pulses of fuel to the various fuel injectors in the engine.
While such fuel distribution valves work well in diesel engines employing conventional pressure fuel distribution systems, the inventors have observed that the fuel distribution shaft in such valves may exhibit excessive wear in high pressure fuel systems, and may even seize or fall over time. The applicants have further discovered that such excessive wear is caused by the high side loading on the fuel distribution shaft that takes place when the fuel pulses are pumped into the valve body at high pressures (i.e., between 20,000 psi and 30,000 psi). The high pressure associated with the discharge of such a fuel pulse causes the side of the fuel distribution shaft opposite the fuel distributing port to push tightly against the inner diameter of the surrounding bore of the valve body, thereby breaking through the film of lubricant normally present. The friction generated from the resulting metal-to-metal contact may cause excessive wear on the shaft, which can ultimately result in shaft seizure and valve failure.
Clearly, there is a need for an improved fuel distribution valve that avoids the problem of excessive valve shaft wear when used in a high-pressure fuel system. Ideally, such a fuel distribution valve should be reliable, simple in structure, and capable of avoiding high side loading of the valve shaft over a broad range of engine speeds. It would further be desirable if such a fuel distribution valve was compatible with a broad range of different diesel engine designs.